Alan’s Outcrop
Near Anaconda, Montana
Day 4, Stop 4

Created by: Sherri Jackson, Gloria Reichmann, Chris Milford

Observations:


Picture 1

This rocky outcrop was found just north of Anaconda off Highway One. Multiple layers uplifted at a 70-degree angle extended approximately 150 meters and had a height of about 100 meters. Small angular fragments were deposited on the shoulder of the road near the base of the hill. The dense fragments showed fine-grained layering.


Picture 2

In some places these lined patterns were found on the rock face. Have you seen anything like this before? Small spherical dots are randomly spaced throughout this section. How many can you count?


Picture 3

The rock was weathering off in thin layers. When examining a thin layer the under side was often red and the top side was gray. When a hammer struck the rock on the hillside a ringing sound was heard.


Picture 4

Some surfaces displayed parallel ridges that extended about 25 meters up the face of the outcrop. Can you observe any patterns here?


Picture 5

Huge fractured blocks are reminiscent of the stones found in the pyramids.

Interpretations:
The rocks we see here were deposited in shallow water (ripples), quiet conditions (fine grained) in an environment that dried out (mud cracks). This may have been a lake environment, or a shallow bay, or tidal flat. As the fine grained sediments dried the mud shrank and cracks were formed. Hexagonal patterns are commonly formed. Trace fossils of raindrop impressions are often found. (See picture 2)

The small patterned ridges are evidence of wave action. They are often referred to as ripple marks, much smaller than the ones found on the Camas Prairie on our first field day.

This rock is referred to as “meta-sedimentary” – it started as a sedimentary rock that was heated up and put under pressure - metamorphosed. It exhibits the characteristic ring of slate when struck with a rock hammer.

Big Picture:
Comparison of Ancient Shallow Lake to Glacial Lake Missoula

Glacial Lake Missoula existed for a short geologic period of time, 10 to 12,000 years ago. It left a sedimentary bed of glacial rock flour in which there was no evidence of life. The level of Lake Missoula changed periodically leaving a record of shorelines.

The Ancient Shallow Lake (or bay) we see at this field stop formed 1.5 billion years ago and created huge layers of sediments through the millennia. There was no evidence of life found in its sediments. The levels of this shallow water environment changed periodically leaving laminations and drying out to form mud cracks.